First-principles Simulations For Liquid And Crystallization Of Phase Change Material Ge-Sb-Te Alloys

With the development of science and technology, as well as the improvement ofdairy life, flash memory has already can’t meet the needs of the development ofelectronic products due to the relatively slow read/write speed and few circle. Thechalcogenide based phase-change memory has been regarded as the promisingcandidate for the next-generation nonvolatile memory due to its excellentcharacteristics such as non-volatile, fast write/read speed（10ns）, good endurance （>1013）, good stability of the data, low power consumption, small size and goodcompatibility with complementary metal-oxide semiconductor （CMOS） technologies.In lots of phase change materials, Ge-Sb-Te alloy which own the large contrast of thereflectivity and resistance between the crystal and amorphous phase has been widelyused in optical disk and nonvolatile memory. In phase change material Ge-Sb-Tealloys, Ge₂Sb₂Te₅was applied frequently. This thesis mainly using the first principlemolecular dynamics investigate the liquid structure of the Ge-Sb-Te alloys andcrystallization of the Ge₂Sb₂Te₅to explore the dynamics during data storage.Firstly, we introduce the phase change memory and phase change material. Sincethe Stanford ovshinsky found that phase change storage materials in the1960s, thephase change storage material has quick development, there are many different kindsof phase change storage materials which have the different properties, then the phasechange memory are also different. Although phase change storage material Ge-Sb-Tealloy has many advantages, but there are also some drawbacks, so many scientificresearchers at home and abroad are working on the problem of the phase changestorage materials to improve their performance in all aspects.Secondly, we introduce the theoretical basics on first-principles calculations,which mainly include the density functional theory. The content of the theory arebriefly introduced including Hohenberg-Kohn theorem and kohn-Sham equation.Thirdly, we used first principle molecular dynamics to simulate the Ge₁Sb₂Te₄,Ge₂Sb₂Te₅and Ge₄Sb₁Te₅liquid phase structure, and compared the liquid properties.From the mean square displacements （MSD）, observed that the element coupled statefor liquid Ge₁Sb₂Te₄and Ge₂Sb₂Te₅is significantly better than that of Ge₄Sb₁Te₅.Detailed analyses by pair correlation functions （PCF） and compositional disordernumbers （CDN） show that Ge₂Sb₂Te₅has the best stability among the three liquids. At last, we used the first principle molecular dynamics to simulate thecrystallization of phase change material Ge2Sb2Te5and analyses the mechanism ofcrystallization. From the bond angle distribution function, pair correlation function,the charge transfer and the number of atom of crystallization, we found in thecrystallization process the Sb atoms play a leading role, and the Ge atoms changesfollowing the Sb elements.The studies here offer some microscopic view on the dynamic process during thephase-change data storage. The melt behavior offers some important suggestions onthe phase stability for special components. The realization of crystallization in MDreveals some of possible reasons for the fast storage. We expect by first-principlescalculation some key physical questions （not easy be discussed in experiment） can beanalyzed and understood in atomic scale for phase-change memory application.